Analog switch circuit having output offset compensation circuit

ABSTRACT

An analog switch circuit which is provided with a transmission gate consisting of a first n channel MOS transistor and a first p channel MOS transistor, which transistors are connected in parallel, wherein the output terminal of said transmission gate is connected to a second n channel MOS transistor and a second p channel MOS transistor, which transistors are supplied with an output voltage Vout from the transmission gate, and wherein mirror capacitances C mP12 , C mP13 , C mN12 , C mN13  are provided at the output terminal of the transmission gate to offset a difference between the mirror capacitance C mN11  of the first n channel MOS transistor and the mirror capacitance C mP11  of the first p channel MOS transistor.

BACKGROUND OF THE INVENTION

This invention relates to an analog switch circuit of a CMOS (complementary metal oxide semiconductor) FET construction.

As shown in FIG. 1, an analog switch circuit of the CMOS type is fundamentally comprised of a transmission gate of the CMOS type. Specifically, the analog circuit is formed of a p channel MOS transistor P₁ and an n channel MOS transistor N₁, which transistors are connected in parallel. An input signal voltage Vin (FIG. 3C) is supplied to an input side node J₁ of the p and n channel MOS transistors P₁, N₁. An output signal voltage Vout (FIG. 3D) is taken out from an output side node J₂. The gate of the n channel MOS transistor N₁ is supplied with a control signal φ (FIG. 3A). The gate of the p channel MOS transistor P₁ is supplied with a control signal φ (FIG. 3B) having a phase which is opposite that of signal φ. A mirror capacitance C_(mN) is present between the output side node J₂ of the n channel MOS transistor N₁ and the p channel MOS transistor P₁, and the gate of said n channel MOS transistor N₁. A mirror capacitance C_(mP) is present between the output side node J₂ and the gate of the p channel MOS transistor P₁. A load capacitance C_(L) is present between the output side node J₂ and ground. FIG. 2 is a curve diagram showing the characteristics of a resistance against the input signal voltage Vin, which resistance arises between the input and output terminals of the subject switch circuit; and the characteristics of the resistance prevailing in the p channel MOS transistor P₁ and n channel MOS transistor N₁. With reference to FIG. 2, the abscissa denotes the input signl voltage Vin, and the ordinate represents a resistance value R. Roman numerals I and II respectively indicate the curves characterizing the resistances prevailing in the p channel MOS transistor P₁ and the n channel MOS transistor N₁. Roman numeral III shows a curve characterizing a resistance prevailing throughout the subject analog switch circuit; or, specifically, the resistance between the input and output terminals thereof. In this case, it is preferred that a resistance value R prevailing between the input and output terminals of the subject analog switch circuit be rendered constant throughout the entire range of the input signal voltage Vin. To stabilize said resistance value R, the conventional practice is to let the p channel MOS transistor P₁ and n channel MOS transistor N₁ have an equal resistance value. To attain this object, the p channel MOS transistor P₁ is chosen in such a way as to have a channel width twice as broad as that of the n channel MOS transistor N₁. However, the conventional procedure has a drawback, in that capacitance C_(mP) assumes a value twice that of capacitance C_(mN) ; and, as a result, the control pulse φ has its voltage level changed from that of the V_(DD) voltage to that of the ground voltage GND, and the control pulse φ has its voltage level varied from that of the ground voltage GND to that of the V_(DD) voltage. Therefore, when the transmission gate is rendered nonconductive, the output signal voltage Vout is offset from a correct value to the extent of ΔV, in accordance with the difference between capacitance C_(mP) and capacitance C_(mN).

The offset voltage V₁ occurring when the control pulse φ has its logic level varied from "0" to "1", and the offset voltage V₂ arising when the control pulse φ has its logic level changed from "1" to "0" are respectively expressed as follows: ##EQU1## Therefore, the output voltage Vout undesirably appears in a form offset to the extent of the sum ΔV of voltages V₁ and V₂, which sum is expressed as: ##EQU2##

SUMMARY OF THE INVENTION

Since this invention has been contrived in view of the above, it is intended to provide an analog switch circuit which comprises a CMOS type transmission gate and is characterized in that, when the transmission gate is rendered nonconductive, it is possible to prevent the offsetting of an output voltage by a difference between the mirror capacitances of the p channel and n channel MOS transistors.

According to the invention, an analog switch circuit is provided, which circuit comprises a transmission gate consisting of a first n channel MOS transistor and a first p channel MOS transistor which are connected in parallel and whose gates are respectively supplied with control signals having opposite logic levels, and a second n channel MOS transistor and second p channel MOS transistor, whose gates are connected to the output terminal of said transmission gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a transmission gate which basically constitutes an analog switch;

FIG. 2 is a curve diagram showing the level of a resistance presented by the transmission gate of FIG. 1 to an input signal voltage;

FIGS. 3A to 3D are time charts of signals appearing in various sections of the transmission gate of FIG. 1; and

FIG. 4 is a circuit diagram of an analog switch embodying this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 4 which shows the circuit arrangement of an analog switch embodying this invention, an n channel MOS transistor N₁₁ and p channel MOS transistor P₁₁ are connected in parallel to constitute a transmission gate TG. An input signal voltage Vin is impressed on the junction J₁ of said transistors N₁₁, P₁₁, which junction faces the input side. An output signal voltage Vout is taken out of the other junction J₂ of said transistors N₁₁ and P₁₁. A control signal φ (FIG. 3A) is supplied to the gate of the n channel MOS transistor N₁₁. A control signal φ (FIG. 3B) having an opposite phase to the control signal φ is delivered to the gate of the p channel MOS transistor P₁₁. A mirror capacitance C_(mN11) is present between that junction J₂ of the parallel connected n channel MOS transistor N₁₁ and p channel MOS transistor P₁₁ which faces the output side of the transmission gate TG and the gate of n channel MOS transistor N₁₁. A similar mirror capacitance C_(mP11) prevails between the junction J₂ facing said output side and the gate of the p channel MOS transistor P₁₁. The gates of an n channel MOS transistor N₁₂ and a p channel MOS transistor P₁₂ are connected to the junction J₂ of the aforesaid transistors N₁₁, P₁₁, which faces the output side of the transmission gate TG. Therefore, an output signal Vout from the transmission gate TG is supplied as a gate signal to the MOS transistors N₁₂, P₁₂. The source and drain of MOS transistor N₁₂ are connected to the gate of MOS transistor P₁₁. The drain and source of MOS transistor P₁₂ are connected to the gate of MOS transistor N₁₁. Therefore, the drain and source of MOS transistor N₁₂ are supplied with a signal φ which is delivered to the gate of the p channel MOS transistor P₁₁ of the transmission gate TG. The drain and source of MOS transistor P₁₂ are supplied with a signal φ which is delivered to the gate of the n channel MOS transistor N₁₁. A mirror capacitance C_(mN12) is present between the gate of the n channel MOS transistor N₁₂ and, e.g., the drain thereof. Further, a mirror capacitance C_(mN13) exists between the gate of said n channel MOS transistor N₁₂ and, e.g., the source thereof. A mirror capacitance C_(mP12) is present between the gate of the p channel MOS transistor P₁₂ and, e.g., the drain thereof. A mirror capacitance C_(mP13) exists between the gate of said p channel MOS transistor P₁₂ and, e.g., the source thereof. A load capacitance C_(L) is present between the aforesaid junction J₂ facing the output side of the transmission gate TG. An output capacitance Cout, as viewed from the output side of the whole circuit of the subject analog switch, is expressed as:

    Cout=C.sub.L +C.sub.mP11 +C.sub.mP12 +C.sub.mP13 +C.sub.mN11 +C.sub.mN12 +C.sub.mN13

Assuming that the channel widths of the respective transistors P₁₁, P₁₂, N₁₁, N₁₂ are indicated by the symbols W_(P11), W_(P12), W_(N11) and W_(N12) ; if, in this case, the respective channel widths are as chosen as to have relationships such as the following:

    W.sub.P12 =W.sub.P11 /2

    W.sub.N12 =W.sub.N11 /2,

The following equations result:

    C.sub.mP12 =C.sub.mP13 =C.sub.mP11 /2

    C.sub.mN12 =C.sub.mN13 =C.sub.mN11 /2.

Therefore, the forming equations are established as follows:

    C.sub.mP12 +C.sub.mP13 =C.sub.mP11                         (1)

    C.sub.mN12 +C.sub.mN13 =C.sub.mN11                         (2)

When the channel widths of the transistors P₁₁, P₁₂, N₁₁, N₁₂ are chosen as described above. The following facts are recognized.

(i) When the control pulse φ has its logic level changed frm "0" to "1", the resultant output voltage Vout indicates an offset voltage V₁ expressed as: ##EQU3##

(ii) When the control pulse φ has its logic level varied from "1" to "0", the resultant output voltage Vout shows an offset voltage V₂ expressed as: ##EQU4##

A composite offset voltage ΔV related to the output voltage Vout is represented by the sum of offset voltage V₁ and V₂, i.e., by a value arrived at by adding equations (3) and (4), and is expressed as follows: ##EQU5## When the above equations (1), (2) are substituted for equation (5), the following equation results:

    ΔV=V.sub.1 +V.sub.2 =O

This means that when, in the analog switch arranged as shown in FIG. 4, the channel widths of the MOS transistors P₁₁, P₁₂, N₁₁, N₁₂ are so chosen as to have such relationships as W_(P12) =W_(P11) /2 and W_(N12) =W_(N11) /2, no offset appears in the output voltage Vout when the transmission gate TG is rendered nonconductive.

The foregoing embodiment of this invention refers to the case wherein the channel width W_(P12) was so chosen as to be half of channel width W_(P11), and channel width W_(N12) was so chosen as to be half of channel width W_(N11). However, channel widths W_(P12) and W_(N12) need not be limited to half the corresponding channel widths (W_(P11) and W_(N11)). If offset voltage can be overlooked to some extent, it is possible to let channel widths W_(P12) and W_(N12) bear any other ratio to the corresponding channel widths W_(P11) and W_(N11). Further, the aforementioned embodiment refers to the case wherein mirror capacitances C_(mN12) and C_(mP12) were respectively chosen to exit between the gates of the MOS transistors N₁₂, P₁₂, and the drains thereof. However, it is also possible to let said mirror capacitances C_(mN12) and C_(mP12) be present between the gates of said MOS transistors N₁₂, P₁₂ and the sources thereof. In the latter case, mirror capacitances C_(mN13) and C_(mP13) are obviously meant to be located between the gates of the MOS transistors N₁₂, P₁₂ and the drains thereof.

It should be noted that this invention is not limited to the above-mentioned embodiment, but can be practiced with various changes and modifications known to those skilled in the art, without departing from the scope and object of the invention.

As mentioned above, this invention provides an analog switch wherein the output side of a transmission gate consisting of a p channel MOS transistor and n channel MOS transistor is provided with a p channel MOS transistor and n channel MOS transistor which are supplied with an output voltage from said output side; and a capacitance is provided on the output side of the transmission gate, to thereby suppress the occurrence of any offset voltage resulting from a difference between the mirror capacitances of the p and n type MOS transistors constituting the transmission gate, when the transmisson gate is rendered nonconductive. 

What is claimed is:
 1. An analog switch circuit responsive to complementary control signals, said analog switch circuit comprising:a transmission gate comprising a first n channel MOS transistor and a first p channel MOS transistor which are connected in parallel to form first and second junctions, said first junction forming an input terminal and said second junction forming an output terminal, the gates of said first n and p channel MOS transistors each being connected to receive a respective one of said complementary control signals; and an output voltage compensation circuit comprising a second n channel MOS transistor and a second p channel MOS transistor whose gates are connected to said output terminal of said transmission gate, the drain and source of said second n channel MOS transistor being connected to said gate of said first p channel MOS transistor, and the drain and source of said second p channel MOS transistor being connected to said gate of said first n channel MOS transistor.
 2. The analog switch circuit according to claim 1, wherein, when the channel widths of the first and second p channel MOS transistors are respectively denoted by W_(P11), W_(P12), and the channel widths of the first and second n channel MOS transistors are respectively represented by W_(N11), W_(N12), the following equations result:

    W.sub.P12 =W.sub.P11 /2

    W.sub.N12 =W.sub.N11 /2 